In such blades, the reinforcing fiber strands extend substantially in radial directions, whereby the fiber composite material encloses a blade core and the fiber strands loop around a shackle member which secures the blade proper to a rotor. German Patent Publication (DE-OS) No. 1,628,286 discloses an axial blower vane or blade which is secured in a bore of a radially extending shaft which in turn is secured to the rotor. This known mounting permits an angular movement of the blade or vane as the rotor rotates in a horizontal plane. The purpose of the free angular movement is to enable the blade or blades to align themselves in the centrifugal force field in such a way that they are not exposed to any bending loads and hence are not subject to any bending stress. Thus, the known construction does not use any flange for taking up bending moments. That type of construction is not suitable for propeller blades which are intended to provide propulsion because the known blade mounting permits the blades to tilt in response to a shearing force caused by the gas flow. Such tilting is undersirable for propulsion propellers because it results in unbalances or flutter vibrations of the propeller.
It is, however, desirable that the blades for modern propulsion plants having a high bypass ratio are constructed as much as possible of fiber composite material because contrary to all-metal blades, this type of construction results in a desired weight reduction and renders the adjustment (tailoring) of the blade's critical vibration modes and frequencies in order to avoid meeting the harmonic vibration orders of the propulsion engine, i.e. critical resonances. Such blades must be mounted to the rotor in a manner stiff against stiff manner to bending but torsionally adjustable. Particularly, prop-fan blades may be advantageously constructed in this manner. However, constructing such blades of fiber composite material poses its own problems due to the high centrifugal forces which cause respective stress in the blades and particularly in the blade roots. The shearing forces caused by the aerodynamic gas flow add to the problems. These problems are aggravated if one has to take into account that foreign objects may enter such a prop-fan propulsion plant, for example, birds may be sucked into such a propulsion plant of an aircraft, whereby these foreign objects impact on the rotor blades. Such impacts cause high bending loads and respective stresses which cannot be taken up by the above known composite propeller blades and by the mounting mechanisms which secure these conventional propeller blades to the rotor.
Austrian Patent (AU-PS) No. 249,847 describes a mechanism in which a rotor blade is secured to a rotor hub by means of a foot plate through which a number of screw bolts extend which are subject to tension stress. This type of arrangement has the disadvantage that the centrifugal and bending forces can be taken up only by the bending of the foot plate flanges. Any additional bending and torsion moments that may occur upon impact of a foreign body, must also be taken up by the same screw bolts.
In this context the fiber composite materials have the disadvantage that due to weak laminar interfaces they tend to delaminate in response to bending loads. Further, the cross-sectional area for taking up loads is small where the mounting screws are located, thus the load transfer and respective stress is concentrated substantially in a point, whereby the screws are exposed to tension stress while the composite material is exposed to compression stress transverse to the fibers. Such stress application is disadvantageous with regard to the abilities of the respective materials. The composite material is capable of taking up longitudiual tension stress but not so capable of taking up transverse compression stress. As an overall result, delamination due to low cycle fatigue and due to high cycle fatigue does occur. In case the mounting fails, the respective blade which is subject to a centrifugal force of more than 15 tons, is thrown off. Thus, heavy weight burst protection features must be employed in the housings of such conventional rotors or in the airframe in order to avoid catastrophic damages.
Another conventional construction is described in an article entitled "Composite Propellers, Some Pros and Cons" in the magazine "Aerospace Engineering", May 1986. The mounting described in the article secures the propeller blade to the hub by means of a clamping mechanism. In this known system both the centrifugal forces and the bending moments are transmitted through the same force retention element in the form of a shearing load applied to the laminations of the composite material. In other words, the resin of the matrix material is primarily subjected to the transmitted stress while the fibers remain relatively unstressed.
German Patent (DE-PS) No. 672,645 discloses a further mechanism in which the centrifugal forces are transferred from a wooden lamination into a hub foot plate by means of steel tension anchors inserted into the wood lamination. This type of structure is not suitable for the relatively slender fiber composite structures of modern supersonic rotor or propeller blades because these profiles are too thin for such steel tension anchors. Adhesion and thermal mismatch problems also would occur.
German Patent Publication (DE-OS) No. 2,832,098 describes yet another mechanism in which the centrifugal forces and bending moments are taken up together by radially extending steel tension anchors. These steel tension anchors are cross connected by shearing bolts which extend perpendicularly through a sandwich type lamination skin of the rotor hub. In such a structure all loads are transmitted from the lamination into the shearing bolts as compression loads and as interlaminar shearing loads. This known load transmission is not advantageous for fiber composites which are better capable of transmitting tension loads. Besides, the known construction does not provide multitude or rather redundant load transmission paths.
Another known unducted Propfan design, disclosed in Aviation Week & Space Technology, Apr. 13, 1987, pp 52-67 and p. 90-93, offers another known solution to the load transfer problem between a carbon fiber composite (CFRP) blade and its retention mechanism.
The blade root is manufactured from titanium metal and comprises an inner titanium spar which extends radially outward into the propeller.
The CFRP composite blade is bonded to this titanium spar like a glove. Thus, all the forces within the CFRP blade are to be transferred through the relatively weak and unreliable bond line, which in itself is prestrained by thermal expansion mismatch (CFRP: .alpha..sub.T .apprxeq.0-2*10.sup.-6 ; Ti: .alpha..sub.T .apprxeq.10*10.sup.-6 (mm/mm.degree. C.). Some "throw-offs" are known to have occurred.
A further known propfan blade uses an aluminium spar running from root to tip of the propeller blade. The spar is surrounded by a fiberglass composite shell filled with low density material. Though no spar is known to have been lost, foreign object impact has caused severe damage to the shell (Aviation Week & Space Technology, Apr. 13, 1987, pp 78-79).